The invention relates to an equipment for driving plate cylinders of a printing machine furnished with a Direct Imaging system, the plate cylinders being coupled on the one hand with a central drive unit by means of a gear wheel assembly with disengageable coupling means and on the other hand with an auxiliary drive unit,
There are known two methods of a digital procedure to prepare a printing plate for an offset printing machine.
The first technology procedure, which is the most applied method, is the computer-to-plate procedure, known as CTP system. The method is carried out outside the printing machine and applies an appropriate CTP equipment comprising an imaging unit and a rotating cylinder with affixed printing plate.
At a given moment the CTP equipment allows only for a single exposition of an image upon a single printing plate. For a print of a multicolour image it is necessary to consecutively expose all printing plates in individual colours for respective printing units and provide each plate with register slots. The completed printing plates are one by one clamped to respective plate cylinders of the printing machine.
The other digital technology is known as DI System, direct imaging procedure, applying laser beam for direct exposition of an image upon a printing plate still accommodated in the printing machine. The exposition is performed simultaneously on all printing units. Rotation of plate cylinders with affixed printing plates is controlled by a central driving unit. During the said procedure, apart from the respective plate cylinder, there are rotating all the cylinders, transport drums and all mechanisms operating during the printing process. A very important parameter for the printing machine with the DI system is the time period necessary for preparation of a new printing plate. The speed of an image transfer upon a printing plate depends on performance of the imaging equipment, namely upon number of spots exposed within a time unit and upon speed of rotation of a plate cylinder, which is driven by the central drive. Due to increased speed of the plate cylinder there are intensified vibrations of all rotating mechanisms of the printing machine. The high rate of vibrations has negative influence upon the quality of an image exposition on the printing plate. To eliminate this undesired feature the image exposition is performed by the plate cylinder speed lower than the one corresponding to the peak operation of the printing machine. The fact that all rotational parts are running during the preparation of printing plates adds to wear of the printing machine. Efforts to cut down the time period for the preparation of printing plates by installation of more powerful imaging equipment have resulted in cost being not proportional to the machine performance.
An equipment for a radial turning and axial shift of a plate cylinder, which is common by conventional printing machines, is not indispensable by machines equipped with the DI system, but application of the equipment helps the personnel to arrange the desired sheet register of colours. In a case the aid equipment is not available with the printing machine furnished with the DI system is not available, by any colour misregister there is necessary a correction of the image exposition to be provided for on a new printing plate. The whole process to make a printing machine ready for a new print is therefore costly and needs a long time to be completed. By known printing machines the equipment for angular displacement is currently mounted at its one side, while the equipment for an axial shift is on the opposite side. Provided it is required to add another mechanism for controlling the plate cylinder, the fact, that both machine sides have been already xe2x80x9coccupiedxe2x80x9d, makes the machine redesign a very complicated affair.
From a paper DE-A-19723147 there is known an equipment for driving cylinders of a printing machine provided with the DI System, where the plate cylinders are coupled with a central drive unit by means of a gear wheel assembly to which the printing cylinder is coupled by disengageable coupling means and the equipment comprises.an auxiliary drive unit coupled with the plate cylinders. The said coupling means comprise a multi-position clutch and a metal-bellows clutch. Such an embodiment is complicated and needs mutual tuning of both clutches to achieve their proper co-operation and reliability of the whole system.
It is an object of the invention to eliminate the above drawbacks and facilitate preparations of new printing plates, especially for multicolour prints. It is a further object of the invention to facilitate adjustment of the register of individual colours.
The foregoing problems are solved and the above objects are achieved by an equipment for driving plate cylinders of a printing machine, the plate cylinders being coupled on the one hand with a central drive unit by means of a gear wheel assembly with disengageable coupling means and on the other hand with an auxiliary drive unit, the equipment in an embodiment in accordance with the present invention comprising the plate cylinder, which is at one end accommodated in a bearing of a print unit side wall by means of a carrier which is axially shiftably seated at a neck of the plate cylinder and is in a direct engagement with the plate cylinder. The carrier is coupled with the auxiliary drive unit by means of a belt drive and with an output wheel of the gear wheel assembly by means of an electrically operated clutch, the clutch engaging with the output wheel and a driven pulley of the belt drive.
According to another aspect of the invention, the equipment comprises a mechanisms for angular displacement and a mechanism for axial shift, both mechanisms being coupled with the plate cylinder. According the first preferred embodiment of this aspect of the invention, the mechanism for angular displacement of the plate cylinder comprises an inner gear wheel swivelably seated on the carrier and coupled with an inner motor. The inner gear wheel is provided with a shoulder having an outer thread, which is screwed into an inner board attached in a constant distance to the side wall of the printing unit. Further in accordance with the second aspect of the invention the preferred embodiment of the mechanism for axial shift of the plate cylinder comprises outer gear wheel coupled with an outer motor. The outer gear wheel is in a swivel seat arranged on a rod rigidly attach to the plate cylinder neck. The outer gear wheel is provided with a shoulder, which is by means of its outer thread sewed into a hole in an outer board, spaced apart from the printing unit side wall at a fixed distance.
The new features provided by the invention comprise elimination of vibration of the rotating plate cylinder during direct imaging procedure of preparation of a printing plate, significant decrease of the time period of the procedure as there can be applied higher plate cylinder speed and there is also achieved a high extend of decrease of wear of rotational parts of the printing machine during the Direct Imaging procedure. It is also possible to prepare printing plates even for conventional printing machines of the size of the printing machine with the DI System.
According to other feature of the invention the compact design of the plate cylinder with the mechanisms for angular displacement and axial shift facilitates corrections of the register of individual colours. The single-sided arrangement of all the mechanisms according the invention makes the other side of the printing machine free for any other control accessories.